1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device. More particularly, it relates to a new and improved semiconductor integrated circuit (IC) device wherein a plurality of field effect transistors (FET's) are formed on a group III-V compound semiconductor wafer.
2. Description of the Related Art
In microelectronics, mainly silicon semiconductor devices are used. Silicon transistor elements have been miniaturized to obtain a high speed operation and integrated density of the semiconductor device.
However, to realize a further improve operation speed than that obtained by the properties of silicon, a compound semiconductor integrated circuit device using gallium arsenide (GaAs), which has an electron mobility (drift velocity) remarkably larger than that of silicon, has been developed.
The present inventors found that, in the III-V group compound, e.g., GaAs, semiconductor, the gate threshold voltage (V.sub.th), K-value (mutual conductance), and piezoelectric polarization due to stress are related to the material and thickness of an insulating layer formed on a gate electrode, and to the crystal surface and zone axis of a semiconductor substrate.